Lead-free piezoelectric crystals grown by the micro-pulling down technique in the BaTiO₃-CaTiO₃-BaZrO₃ system

BaTiO₃-based crystal fibres with mm-sized grains were grown by the micro-pulling down technique from the BaTiO₃-CaTiO₃-BaZrO₃ solid solution with pulling velocities of about 6, 9 and 15 mm h^-1. The natural growth direction was identified as (001)_pc. For the pulling velocities of about 15 mm h^-1 and 9 mm h^-1, effective partition coefficients have been calculated from Castaing micro-probe measurements, and gave, respectively, 1.3 and 2 for Zr, and 0.95 and 0.9 for Ca. Laser-induced breakdown spectroscopy measurements reveal a strong inhomogeneity and variations of Zr contents while Ca contents show an opposite variation trend with a more steady distribution. Coexistence of two crystallized perovskite solid solutions is suggested. Most efficient polycrystals with mm-sized grains and 0.5 mol% Zr and 11 mol% Ca as average contents exhibit Curie temperatures higher than 113 °C, electromechanical coupling factors k_t up to 41% and piezoelectric charge coefficients d₃₃ up to 242 pC N^-1 at room temperature. These values are similar to piezoelectric coefficients reported in the literature for oriented flux-grown single crystals with close compositions. Both chemical and physical results obtained in the BCTZ system make the μ-PD technique a promising way to improve the piezoelectric response of lead-free solid solution-based single crystals.